Distillate feedstocks that have significant concentrations of organic nitrogen and polyaromatic compounds are more difficult to upgrade by the removal of the nitrogen and polyaromatic compounds in order to provide a saleable product than it is to upgrade distillate feedstocks of which the primary concern is the removal of organic sulfur and monoaromatic compounds.
In recent years, distillate product quality specifications have become more stringent, which have made it more difficult to meet the quality specifications with existing processing schemes. Due to these new, more stringent product specifications, existing processes are required to be modified so as to be able to process the distillate feedstocks to yield products that meet the specifications. Also, it is desirable to develop new processes that can provide for the manufacture of distillate products which meet the more stringent standards. For diesel fuel products, the quality correlates with the Cetane Index. Generally, it is desired to have a high Cetane Index that is preferably greater than 40. The value of the Cetane Index for diesel fuel tends to negatively correlate with the aromatics concentration level with higher concentrations of aromatics tending to lower the Cetane Index and lower concentrations tending to increase the Cetane Index.
In the processing of diesel feedstocks, it typically is more difficult to convert or remove concentrations of polyaromatics than it is to convert or remove comparable concentrations of monoaromatics, and it is also more difficult to convert or remove concentrations of organic nitrogen than it is to convert or remove comparable concentration levels of organic sulfur.
One process for producing a low sulfur diesel product with a high cetane number is disclosed in U.S. Pat. No. 7,790,020. In this process, a diesel feed is first subjected to a hydrodesulfurization step, operated at low-pressure conditions, with a minimal saturation of aromatics. The effluent from the desulfurization zone is then introduced into a separation zone whereby it is separated into a vapor stream and a liquid hydrocarbon stream. The liquid hydrocarbon stream is admixed with hydrogen and the admixture is passed to a substantially liquid-phase continuous reaction zone that is operated at high-pressure conditions significantly above those of the hydrodesulfurization step to provide for the saturation of aromatics and to yield an effluent having an improved cetane number of at least 40. Due to the operation of the hydrotreating zone at a low pressure, a small, low-pressure recycle compressor, instead of a high-pressure recycle compressor, is used to recycle hydrogen to the first stage hydrodesulfurization zone. There is no mention of hydrodenitrogenation or partial saturation of polyaromatics to monoaromatics as taking place in the first step of the process. Due to the low-pressure operation of the first step, it would be expected that no significant hydrodenitrogenation of a feedstock having a high organic nitrogen concentration would occur. It is further noted that there is no mention of the use of multiple catalyst beds contained within a single reactor vessel or the use of interbed quenching.
Another process disclosed in the art for the hydrotreating of middle distillate feeds to produce a low-sulfur and low-aromatic diesel product is described in U.S. Pat. No. 5,110,444. This process employs three reaction zones in series with the first two reaction zones intended to provide a high degree of desulfurization and the third reaction zone intended to provide a high degree of aromatics saturation. The hydrocarbons leaving the first and second reaction zones are subjected to countercurrent stripping with hydrogen to remove hydrogen sulfide prior to passage into the next reaction zone. The first reaction zone employs a desulfurization catalyst that comprises nickel and molybdenum or a cobalt and molybdenum on a support. The second reaction zone provides a mild desulfurization and utilizes a noble metal catalyst. The second reaction zone is maintained at desulfurization conditions similar to those of the first reaction zone, but it is operated at a higher pressure and lower temperature. The third reaction zone is a hydrogenation zone that contains a catalyst comprising a noble metal on an inorganic support. The reaction conditions of the third reaction zone are maintained to provide for the saturation of a substantial portion of the aromatic hydrocarbons present in the entering materials, with a low hydrogen sulfide concentration and at the highest pressure and lowest temperature of the three reaction zones of the process.
U.S. Pat. No. 5,114,562 discloses a process for hydrotreating middle distillate feeds to produce a low-sulfur and low-aromatic product. The process of U.S. Pat. No. 5,114,562 utilizes two reaction zones in series instead of three reaction zones in series as in the process of U.S. Pat. No. 5,110,444. The first reaction zone is intended to provide a high degree of desulfurization, and the second reaction zone is intended to provide a high degree of aromatics saturation. The effluent from the first reaction zone is purged of hydrogen sulfide by countercurrent stripping with hydrogen prior to passage to the second reaction zone. The first reaction zone uses a desulfurization catalyst comprising nickel and molybdenum or cobalt and molybdenum on a support, and the second reaction zone uses a noble metal hydrogenation catalyst that comprises platinum or palladium on alumina.
Although there are a wide variety of process flow schemes, operating conditions and catalysts that are used in the processing of middle distillate feedstocks to make diesel products, there is always a desire to provide new and more economical or better methods of manufacturing diesel products. In many cases, even minor variations in process flows or operating conditions or in the catalyst used can have significant effects on process performance and the quality of the end-products.